1. Field of the Invention
The present invention relates to a high pressure processing apparatus and method which employs a high pressure processing fluid. More particularly, the present invention relates to a high pressure processing apparatus and method for subjecting a substrate to a predetermined high pressure process (e.g., for removing any unwanted matter adhered on the substrate) which supplies a high pressure fluid over a substrate, such as: a semiconductor substrate; a substrate for FPDs (Flat Panel Displays) (e.g., a glass substrate for a liquid crystal display device); a glass substrate for a photomask, a substrate for an optical disk, or the like (hereinafter collectively referred to as “substrates”). Furthermore, the present invention relates to a high pressure processing apparatus and method which can be used in a drying process for removing moisture off a substrate surface, or a development process for removing unwanted portions from a substrate surface.
2. Related Art Statement
In recent years, the trend away from using flon (fluorine) for washing substrates having electronic or other components formed thereon has directed much attention to the use of a low-viscosity processing fluid which is kept in a high-pressure state (e.g., super critical CO2) as a release agent or a rinse agent.
Moreover, the need for downsizing (“shrinking”) semiconductor devices in recent years has led to the use of finer design rules (technology node) for devices, and this trend only appears to be growing. Such semiconductor device structures incorporate very minute trenches and holes, both of which need washing. Minute trenches may be employed for capacitors (or the capacitive portions thereof), horizontal wiring (or two-dimensional wiring), and the like. Minute holes may be employed for vertical wiring (three-dimensional wiring: connections between horizontal wires, gate electrode connections for transistors, etc.), and the like.
In such minute structures, increasingly larger ratios between the width and depth thereof (so-called “aspect ratio”) have been used. In other words, there is a tendency to form narrower but deeper trenches, and to form smaller-diameter but deeper holes. Some micro-structures may have a width or a diameter on the order of submicrons, with an aspect ratio exceeding ten. After such micro-structures are formed on a semiconductor substrate through dry etching, not only the upper planar surface, but also the side walls and bottoms of the trenches and holes will be left with contamination, such as residues of the resist, denatured resist resulting from the dry etching, compounds of the resist and the bottom metal, and/or oxidized metals.
Conventionally, such contamination is washed away by using a solution-type chemical. However, since the ingression of a chemical and the later substitution with pure water may not occur smoothly in such micro-structures, unsatisfactory washing results may be obtained. Moreover, although low-dielectric constant materials (so-called “Low-k materials”) are used in order to prevent delay in electrical signals due to the wiring being affected by etched insulative substances, the presence of chemicals tends to ruin the low dielectric constant. In the case where a wiring metal is exposed, it is impossible to employ a chemical which dissolves metals, which in itself is another limitation.
Super critical fluids (SCFs) are considered as a promising alternative for the washing of such micro-structures on semiconductor devices. As represented by a hatched portion in FIG. 8, an “SCF” refers to a substance which is in a state that only exists at a pressure equal to or greater than a critical pressure Pc and at a temperature equal to or greater than a critical temperature Tc. An SCF has properties intermediate between those of liquid and gas, and therefore is suitable for washing on a micro scale. Specifically, an SCF is effective for the washing of organic components due to its density (which approximates that of liquid) and high solubility, enables uniform washing due to its diffusibility which compares that of gas, and is suitable for the washing of micro components due to its low viscosity which compares that of gas.
As a substance to be converted into an SCF, carbon dioxide (CO2), water (H2O), nitrous oxide (N2O), ammonia, ethanol, or the like may be used. Among others, CO2 is frequently used because it can easily attain a super critical state due to its critical pressure Pc being 7.4 MPa and its critical temperature Tc being about 31° C. and because CO2 is non-toxic.
Although CO2 SCF is inert by nature, fluidic CO2 has a dissolving ability similar to that of hexane, and therefore can easily remove moisture, fat, etc., off a substrate surface. Moreover, amines, ammonium fluoride, or the like—which are employed for washing contamination off semiconductor substrates—can be admixed in a suitable concentration range to obtain a multi-component SCF. Such a multi-component SCF is capable of entering into minute device structures to remove the contamination. Moreover, the admixed amines or ammonium fluoride can be easily removed from the minute device structures together with the contamination.
Unlike a solution-type chemical, an SCF does not leave its residues after permeating a low-dielectric constant insulative substance, and therefore does not alter the properties of the insulative substance. Therefore, an SCF is highly suitable for the washing of micro-structures on semiconductor devices.
FIG. 9 illustrates an exemplary apparatus which performs a wash process for a substrate using an SCF. The high pressure processing apparatus shown in FIG. 9 comprises: a cylinder 201 containing liquid CO2; a condenser 202; a booster 203; a heater 204; a substrate washing chamber 205; a decompressor 207; a separation/recovery bath 208; and valves V1 and V2.
Hereinafter a wash operation of a high pressure processing apparatus having the above-described configuration will be briefly described.
First, a substrate as an object to be washed is placed within the substrate washing chamber 205, and the substrate washing chamber 205 is sealed. The following wash process begins after the placement of the substrate. First, the liquefied CO2 in the cylinder 201 is supplied to the condenser 202 so as to be stored there in the liquid state. The liquefied CO2 is compressed by the booster 203 to a pressure equal to or greater than the critical pressure Pc, and is further heated by the heater 204 to a temperature equal to or greater than the critical temperature Tc, thereby being converted into super critical CO2, which is supplied to the substrate washing chamber 205. In the substrate washing chamber 205, a washing takes place by allowing the super critical CO2 to come into contact with the substrate.
The super critical CO2, containing contaminants from the substrate washing (e.g., organic substances, inorganic substances, metals, particles, and/or water which have parted from the substrate and strayed into the super critical CO2 during the washing), is subjected to a final decompression by the decompressor 207 so as to be vaporized. Thereafter, the super critical CO2 is separated into gaseous CO2 and the contaminants in the separation/recovery bath 208. The isolated contaminants are discharged, whereas the CO2 gas is recovered for recycling in the condenser 202. The substrate washing is completed by repeating the above wash process for a predetermined amount of time or longer.
However, in accordance with the above-described conventional high pressure processing apparatus, the surrounding air may stray into the chamber through the hatch opening while positioning the substrate in the substrate washing chamber 205. Therefore, when the SCF which has been used in a wash process is recovered for recycling, the surrounding air components which have strayed into the substrate washing chamber 205 may enter the SCF generation/recovery line and deteriorate the purity of the SCF used for washing.
Even if the substrate washing chamber 205 is installed in a clean room when using the above-described high pressure processing apparatus for washing a semiconductor substrate, the air within the clean room may contain various contaminants, such as SOx, NOx, siloxanes, boron, and vaporous organic substances.
The reduced purity of the SCF may affect the condensation temperature of the CO2 gas which is recovered for recycling, whereby the performance of the substrate washing employing super critical CO2 may be deteriorated.
This problem is true not only of washing techniques employing SCF, but also of any high pressure process, such as development, washing, or drying of a substrate within a closed processing chamber, that employs a subcritical fluid or a high pressure gas of ammonia, for example.
As used herein, a “subcritical fluid” generally refers to a liquid which is in a high-pressure state below the critical point shown in FIG. 8. Fluids which fall within this region are sometimes distinguishable from SCFs. However, since physical properties such as density only undergo gradual (i.e., not stepwise) changes, there may be no physical breakpoint. Therefore, a subcritical fluid might also be usable as an SCF. Any substance which lies in the subcritical region, or more broadly, in the super critical region near the critical point, may sometimes be referred to as a “high-density liquefied gas”.
Thus, a high pressure processing apparatus employing such a high pressure fluid still admits of improvement in the manner of recovering for recycling the high pressure process fluid which has been used in the processing, in terms of preventing deterioration of the process performance.
An apparatus having the configuration shown in FIG. 10 may alternatively be used as an apparatus for performing a wash process for a substrate employing an SCF. The high pressure processing apparatus shown in FIG. 10 comprises: a cylinder 201 containing liquid CO2, a condenser 202, a booster 203, a heater 204, a substrate processing chamber (SCF chamber) 205, a circulator 206, a decompressor 207, a separation/recovery bath 208, a switching section 209, a mixer 210, and a chemical supply section 211 which is coupled via a valve V3.
Hereinafter, a wash operation performed by the high pressure processing apparatus of the above configuration will be briefly described. A substrate as an object to be washed is placed within the substrate washing chamber 205, and the substrate washing chamber 205 is sealed. Awash process as follows is begun after the placement of the substrate. First, the liquefied CO2 in the cylinder 201 is supplied to the condenser 202 so as to be stored there in the liquid state. The liquid CO2 is compressed by the booster 203 to a pressure equal to or greater than the critical pressure Pc, and is further heated by the heater 204 to a temperature equal to or greater than the critical temperature Tc, thereby being converted into super critical CO2, which is supplied to the mixer 210. The mixer 210 mixes a predetermined chemical which is supplied via the valve V3 with the super critical CO2, and outputs the resultant mixture to the substrate washing chamber 205.
The reason for employing the aforementioned chemical will be described. Although the fluidic CO2 has a dissolving ability similar to that of hexane and therefore can easily remove moisture, fat, etc., off the substrate surface, it does not provide a sufficient dissolving ability for high-molecular-weight contaminants such as resists or etching polymers. Therefore, it is difficult to release and remove contaminants by using CO2 alone. This is the reason why a certain chemical (or assistant) is added to the CO2 to assist in the releasing and removal of the high-molecular-weight contaminants.
In the substrate washing chamber 205, a washing takes place by allowing the super critical CO2 to come into contact with the substrate. Specifically, the substrate washing is achieved by allowing the super critical CO2 mixed with the chemical to circulate for a predetermined of time, based on the switching of the switching section 209 and activation of the circulator 206. The circulation-based washing for the substrate is adopted in order to minimize the amount of super critical CO2 used, and to reduce the time required for washing. As a result, the running cost can be curtailed, thereby making for a more economical processing.
The super critical CO2 mixed with the chemical, having dissolved or dispersed therein the containing contaminants from the substrate washing (e.g., organic substances, inorganic substances, metals, particles, and/or water which have parted from the substrate and strayed into the super critical CO2 during the washing), is vaporized and subjected to a final decompression by the decompressor 207 so as to be vaporized. Thereafter, the super critical CO2 is separated into gaseous CO2, the chemical, and the contaminants in the separation/recovery bath 208. The isolated chemical and contaminants are discharged, whereas the CO2 gas is recovered for recycling in the condenser 202. The substrate washing is completed by repeating the above wash process for a predetermined amount of time or longer.
However, in order to use the high pressure processing apparatus for long periods of time, it becomes necessary, after every wash process, to clean the entire system of the chemical and residues in the channels of the circulation line and other components. Moreover, in the case of performing wash processes using different chemicals with the same apparatus, it is also necessary to perform a cleaning process to remove the residues of the chemical used in the previous process, before a new chemical can be used. This cleaning process is usually performed by allowing an SCF to flow through the entire system without mixing any chemicals therein. Therefore, in order to clean the circulation channel 212 which is part of the circulation line, only an SCF is circulated, and after the lapse of a predetermined period of time, the SCF in the circulation line is discharged to the decompressor 207. This operation must be repeated as necessary.
Cleaning the entire system through the above-described operation will invite a prolonged cleaning process time, a lower throughput of the high pressure processing apparatus, and a larger amount of SCF being used in the cleaning process, thus leading to increased cost.
Furthermore, unlike the processing operation performed by the high pressure processing apparatus, the above-described cleaning process is a separately and non-routinely performed process, and therefore does not make for much improved cleanliness within the circulation line. Consequently, the cleanliness with respect to the object to be processed is also deteriorated. Moreover, when wash processes are performed with different chemicals, a chemical which was used before the cleaning process may inevitably be mixed with a new chemical used in the circulation line, thereby resulting in unwanted chemical reactions between the chemicals, or making it impossible to perform a desired wash process. Thus, there are limits to the chemicals which can be used in the conventional high pressure processing apparatus.
Another known method is illustrated in FIG. 11, under which a cleaning process is performed in a conventional high pressure processing apparatus by supplying an SCF containing no chemicals (referred to as “fresh SCF”) to the circulation line from a separate line. In the high pressure processing apparatus shown in FIG. 11, a “fresh” super critical CO2 is supplied from a fresh SCF supply section 213. Therefore, the cleanliness within the substrate washing chamber 205 is improved. However, as is the case with the aforementioned cleaning operation, the cleaning of the interior of the circulation line in this case occurs as a restricted process which requires the entire system to only execute a cleaning operation. Thus, this method does not solve the aforementioned problems.
Likewise, the above problems are true not only of washing techniques employing SCF, but also of any high pressure process, such as development, washing, or drying of a substrate within a closed processing chamber, that employs a subcritical fluid or a high pressure gas of ammonia, for example.